Stents are endovascular prostheses (endoprotheses) or implants that can be used to treat stenoses (vasoconstrictions). They usually comprise a body in the form of a hollow-cylindrical or tubular matrix lattice that is open at both longitudinal ends of the tubes. The tubular matrix lattice of an endoprosthesis of this type is inserted into the vessel to be treated, and is used to support the vessel. Further fundamental body shapes are also possible. Furthermore, the present invention relates to implants that can be used in the field of orthopedic surgery e.g. for the skull region, and, in particular, to implants that have low x-ray visibility due to their small size and wall thickness.
The invention can also be used for stents in neurovascular applications. In that case, absorbable Mg stents are used to hold open the blood vessels that supply the brain. These systems are used in the field of preventing acute ischemic strokes.
Stents or other implants often contain metallic materials in their body. The metallic materials can form a biodegradable material, although they can also contain polymeric, biodegradable materials.
“Biodegradation” is understood to mean hydrolytic, enzymatic, and other metabolic degradative process in the living organism, which are caused primarily by the bodily fluids that come in contact with the endoprothesis and result in a general disintegration of at least large portions of the implant. The term “biocorrosion” is often used as a synonym for the term “biodegradation”. The term “bioresorption” includes the subsequent resorption of the degradative products by the living organism. The objective of using biodegradable implants is for them to be broken down by the organism at a point in time when they are no longer needed e.g. for their supporting effect, and therefore do not remain in the organism as a foreign object for any longer than necessary.
Materials (basic material) that are suitable for the body of biodegradable implants can be composed of one material or a plurality of materials. Examples of suitable polymeric compounds are polymers of the group cellulose, collagen, albumin, casein, polysaccharide (PSAC), polylactic acid (PLA), poly-L-lactide (PLLA), polyglycol (PGA), poly(D,L-lactide-co-glycolide) (PDLLA-PGA), polyhydroxybutyric acid (PHB), polyhydroxy valeric acid (PHV), poly(alkyl carbonate), poly(orthoester), polyethylenterephtalat (PET), polymalonic acid (PML), polyanhydride, polyphosphazene, polyamino acids and their copolymers, and hyaluronic acid. Depending on the properties that are desired, the polymers can be present in pure form, in derivatized form, in the form of blends, or as copolymers. Metallic biodegradable materials are based on alloys of magnesium, iron, zinc, and/or tungsten.
The present invention relates to implants having a body, the material of which contains a metallic material. Further metallic materials in addition to the above-mentioned biodegradable materials are feasible for use. In particular, the present invention relates to implants having the component magnesium, which preferably forms the main component of the body material.
The position of a stent or other implants is often determined using imaging methods e.g. using an x-ray device. Due to the small atomic number and the low density of the biodegradable material magnesium and its alloys, the x-ray visibility of the medical implants made of these materials is very low. To eliminate this disadvantage, it is known to provide medical devices with functional elements that have a material composition in at least a portion of their volume that differs from the material of the body. The so-called “(x-ray) markers” or functional elements contain a material, in particular, that absorbs x-rays and/or other electromagnetic rays to a greater extent (referred to below as x-ray opaque or radio-opaque material) than the material of which the body is composed.
Publication DE 10 2006 038 238 A1 describes an x-ray marker for medical implants made of a biocorrodible metallic material, and a medical implant having an x-ray marker of this type, wherein the x-ray marker is a boride or carbide of the elements tantalum or tungsten. A marker thusly composed has low x-ray absorption, however, since the mean atomic number and the density of these materials are low.
Publication US 2009/0204203 A1 discloses an implant in the form of a bioabsorbable stent having one or more radio-opaque markers. A receiving device (marker support) on the body of the stent, in which the marker is fastened, is passivated or oxidized before the marker is applied. This slows the corrosion of the receiving device, thereby resulting in improved endothelialization.
Furthermore, publication DE 10 2008 054 845 A1 proposes a mechanical anchoring of tantalum- or tungsten markers to magnesium stents, followed by plasma-chemical treatment of this bond. The method described therein has the disadvantage, however, that the mechanical anchoring of markers e.g. by riveting or laser- or electron-beam welding is technologically highly complex and requires extreme fine-motor skills and/or complex and therefore expensive handling techniques. Furthermore, after the passivation layer has degraded, the metallic contact of tantalum or tungsten to magnesium, even if the latter has been initially passivated from the outside, results in accelerated degradation since the materials of the x-ray marker form a local cell with the body material.
As described above, in the case of implants having a body composed of a metallic material, the problem arises with the disposition of metallic functional elements on the body that contact corrosion can occur in the contact region between the material of the body and the material of the functional element. This results in accelerated degradation, or in the separation of the functional element from the body, and therefore the implant is either no longer capable of performing its function, or it cannot be found. The above-described devices from the prior art contain no solutions, or inadequate solutions, to the problem described herein.